An autoradiographic study of incorporation of labelled 5-hydroxytryptophan and 5-hydroxytryptamine in the brain

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In order to investigate the histochemical localization of serotonin in the central nervous system, light and electron microscopic autoradiography has been studied by using labelled 5-hydroxytryptophan (5-HTP) and 5-hydroxytryptamine (5HT).Light microscopic autoradiography was carried out as follows: C14-5-HTP (0.5μc/g) was injected intraperitoneally to the mice pretreated with Catron (0.01mg/g) 5 hours before the administration of C14-5-HTP. C14-5-HT (0.5μc/g) was injected intraperitoneally to the another group of mice without pretreatment. The animals were sacrificed at various time intervals up to 24 hours after injection of labels and autoradiographic preparations were made with kodak NTB2 by dipping method.For electron microscopic autoradiography, H3-5-HTP (5μc/g) was injected intraperitoneally to the rats. One hour after the administration of labelled 5-HT, the rat brain was fixed by OsO4 immersion after glutaraldehyde perfusion and ultrathinsections were coated with Sakura NR-MI by wire-loop method.An autoradiograph of brain after intraperitoneal injection of C14-5-HT revealed that 5-HT was taken up by the paraventricular structures and the neighbouring small area, but not by the ordinary brain regions. An autoradiographic localization of labels following intraperitoneal injection of 4-5-HTP was revealed markedly in autonomic area and limbic system, in addition to paraventricular structures. In as short a time as 5 minutes, marked uptake of C14-5-HTP was observed in the neuropil, while less uptake in the cytoplasm and nuclei. Because the majority of labels in the neuropil is supposed to be due to 5-HT, 5-HT may probably be synthesized and matabolized in the synaptic nerve endings, probably not in the cytoplasm of the cell bodies. Electron microscopic autoradiograph of the nucleus caudatus and nucleus ventromedialis hypothalami one hour after intraperitoneal administration of H3-5-HTP showed that two-thirds of the total silver grains were located in the neuropil and a third in the cell bodies. In the neuropil a half of the grains was localized in the presynaptic nerve endings, having a tendency to be concentrated in the presynaptic terminals containing granulated vesicles.